Dual-loop automatic frequency control for wireless communication

ABSTRACT

Techniques for performing frequency control using dual-loop automatic frequency control (AFC) are described. The dual-loop AFC includes an inner loop that corrects short-term frequency variations (e.g., due to Doppler effect) and an outer loop that corrects long-term frequency variations (e.g., due to component tolerances and temperature variations). In one design, a first inner loop is implemented for frequency control of a first system (e.g., a broadcast system), a second inner loop is implemented for frequency control of a second system (e.g., a cellular system), and at least one outer loop is implemented for adjusting a reference frequency used to receive signals from the first and second systems. Each inner loop estimates and corrects the frequency error in an input signal for the associated system and may be enabled when receiving the input signal from the system. The reference frequency may be used for frequency downconversion, sampling and/or other purposes.

The present application claims priority to provisional U.S. ApplicationSer. No. 60/657,839, entitled “Method and Apparatus for Dual-LoopAutomatic Frequency Control,” filed Mar. 1, 2005, and U.S. applicationSer. No. 60/660,914, entitled “Automatic Frequency Controller,” filedMar. 11, 2005, both assigned to the assignee hereof and incorporatedherein by reference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to automatic frequency control (AFC) for wirelesscommunication.

II. Background

In wireless communication, a transmitter modulates data onto a radiofrequency (RF) carrier signal to generate an RF modulated signal that ismore suitable for transmission. The transmitter then transmits the RFmodulated signal via a wireless channel to a receiver. The transmittedsignal may reach the receiver via one or more propagation paths, whichmay include a line-of-sight path and/or reflected paths. Thecharacteristics of the wireless channel may vary over time due tovarious phenomena such as fading and multipath. Consequently, thetransmitted signal may experience different channel conditions and maybe received with different amplitudes and/or phases over time.

The receiver receives the transmitted signal, downconverts the receivedsignal with a local oscillator (LO) signal, and processes thedownconverted signal to recover the data sent by the transmitter. Thereceiver typically performs frequency control (e.g., frequencyacquisition and tracking) to estimate the frequency error in the LOsignal and to correct this frequency error. This frequency error may bedue to various factors such as receiver circuit component tolerances,temperature variations, and Doppler effect due to movement by thereceiver and/or transmitter. The frequency control may be challenging ifthe requirements on frequency accuracy are stringent. The frequencycontrol may also be complicated if the receiver intermittently receivesdata from the transmitter.

There is therefore a need in the art for techniques to expeditiously andreliably perform frequency control for wireless communication.

SUMMARY

Techniques for performing frequency control using dual-loop AFC aredescribed herein. The dual-loop AFC includes an inner loop that correctsshort-term frequency variations (e.g., due to Doppler effect) and anouter loop that corrects long-term frequency variations (e.g., due tocomponent tolerances and temperature variations). These techniques maybe used for frequency control when receiving one or multiplecommunication systems, e.g., a broadcast system, a cellular systemand/or a satellite positioning system. These techniques may also be usedfor frequency control when receiving a bursty transmission.

In an aspect, the dual-loop AFC is used for frequency control ofmultiple systems. In an embodiment, a first inner loop is implementedfor frequency control of a first system (e.g., a broadcast system), asecond inner loop is implemented for frequency control of a secondsystem (e.g., a cellular system), and at least one outer loop isimplemented for adjusting a reference frequency used to receive thefirst and second systems. The reference frequency may be generated by areference oscillator (e.g., a TC-VCXO) and may be used for frequencydownconversion, sampling and/or other purposes. The first inner loopestimates and corrects the frequency error in a first input signal forthe first system. The second inner loop estimates and corrects thefrequency error in a second input signal for the second system. Thefirst and second inner loops may be enabled when receiving the first andsecond systems, respectively. Separate first and second outer loops maybe implemented for the first and second systems, respectively, and oneouter loop may be selected to update the reference frequency.Alternatively, a single outer loop may be implemented for both systemsand may be updated with first and/or second inner loop. Exemplarydesigns of the inner and outer loops are described below.

In another aspect, the dual-loop AFC is used for frequency control of abursty transmission in which data is received in bursts. In anembodiment, an AFC inner loop is updated in each inner loop updateinstant during each burst of data, and an AFC outer loop is updated ineach outer loop update instant. The inner loop estimates and correctsthe frequency error in the bursts of data. The outer loop estimates andcorrects the frequency error in a reference frequency used to receivethe bursts of data. The inner loop may be updated, e.g., with each OFDMsymbol received during a burst of data. The outer loop may be updated,e.g., at the end of each burst of data.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout.

FIG. 1 shows a terminal communicating with multiple systems.

FIG. 2 shows an exemplary super-frame structure.

FIG. 3 shows a block diagram of the terminal.

FIG. 4 shows a block diagram of an AFC unit.

FIG. 5 shows a block diagram of the dual-loop AFC for one system.

FIG. 6 shows a block diagram of an initial frequency error estimator.

FIG. 7 shows a block diagram of a frequency error estimator.

FIG. 8 shows a model of the dual-loop AFC.

FIG. 9 shows a process to perform frequency control for multiplesystems.

FIG. 10 shows an apparatus to perform frequency control for multiplesystems.

FIG. 11 shows a process to perform frequency control for one system.

FIG. 12 shows an apparatus to perform frequency control for one system.

FIG. 13 shows a process to perform frequency control for bursty data.

FIG. 14 shows an apparatus to perform frequency control for bursty data.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a terminal 110 capable of communicating with multiplecommunication systems. These systems may include a cellular system 120,a broadcast system 130, a satellite positioning system 140, a wirelesslocal area network (WLAN) system (not shown in FIG. 1), other systems,or any combination thereof.

Cellular system 120 may be a Code Division Multiple Access (CDMA)system, a Time Division Multiple Access (TDMA) system, a FrequencyDivision Multiple Access (FDMA) system, an Orthogonal Frequency DivisionMultiple Access (OFDMA) system, a Single-Carrier FDMA (SC-FDMA) system,or some other cellular system. A CDMA system may utilize a radiotechnology such as cdma2000, Wideband-CDMA (W-CDMA), and so on. cdma2000covers IS-95, IS-2000 and IS-856 standards. A TDMA system may utilize aradio technology such as Global System for Mobile Communications (GSM),Digital Advanced Mobile Phone System (D-AMPS), and so on. D-AMPS coversIS-136 and IS-54 standards. These various systems, radio technologies,and standards are known in the art. Cellular system 120 may be aUniversal Mobile Telecommunication System (UMTS) that implements W-CDMA,a CDMA2000 1× system that implements IS-2000 and/or IS-95, a CDMA20001xEV-DO system that implements IS-856, a GSM system, or some othersystem.

Broadcast system 130 may be a MediaFLO system, a Digital VideoBroadcasting for Handhelds (DVB-H) system, an Integrated ServicesDigital Broadcasting for Terrestrial Television Broadcasting (ISDB-T)system, or some other broadcast system. These broadcast systems areknown in the art.

Satellite positioning system 140 may be the United States GlobalPositioning System (GPS), the Russian Glonass system, the EuropeanGalileo system, or some other satellite positioning system. GPS is aconstellation of 24 well-spaced satellites plus some spare satellitesthat orbit the earth. Each GPS satellite transmits an encoded signalthat allows receivers on earth to accurately estimate their positionsbased on measurements for a sufficient number of satellites (typicallyfour) and the known locations of these satellites.

As shown in FIG. 1, cellular system 120 includes a number of basestations 122 that support communication for the terminals within theircoverage. A base station is typically a fixed station used forcommunicating with the terminals and may also be called a basetransceiver subsystem (BTS), a Node B, an access point, and so on.Broadcast system 130 includes a number of broadcast stations 132 thatbroadcast data to the terminals within their coverage. Base stations 122and broadcast stations 132 may be located at different sites (as shownin FIG. 1) or co-located at the same site (not shown in FIG. 1).Satellite positioning system 140 includes a number of satellites 142that transmit signals used for position determination.

Terminal 110 may be fixed or mobile and may also be called a mobilestation, a user equipment, a mobile equipment, and so on. Terminal 110may be a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a wireless communication device, a handheld device, a subscriberunit, and so on. For clarity, much of the following description is foran embodiment in which terminal 110 is capable of communicating with aCDMA system (which may be a UMTS system or a CDMA 1× system), theMediaFLO system, and GPS.

FIG. 2 shows an exemplary super-frame structure 200 that may be used forbroadcast system 130. In the embodiment shown in FIG. 2, thetransmission timeline is partitioned into super-frames, with eachsuper-frame having a particular time duration, e.g., approximately onesecond. Each super-frame includes a field 212 for a time divisionmultiplexed (TDM) pilot, a field 214 for overhead/control information,and a field 216 with N frames for traffic data, where N≧1. A super-framemay also include different and/or additional fields not shown in FIG. 2.

In the embodiment shown in FIG. 2, the TDM pilot is composed of Sidentical pilot sequences, with each pilot sequence containing Ltime-domain samples, where S>1 and L>1. The TDM pilot may be generatedby (1) performing an L-point inverse fast Fourier transform (IFFT) on Lmodulation symbols to obtain a pilot sequence with L time-domain samplesand (2) repeating the pilot sequence S times. The TDM pilot may be usedfor signal detection, frame synchronization, initial frequency errorestimation, coarse time synchronization and/or other purposes.

The overhead information may convey the identity of a broadcast stationtransmitting the overhead information, where and how data channels aresent in the frames of a super-frame, and/or other information. The datachannels are sent in the N frames and at frequency and time locationsindicated by the overhead information. Each data channel may carry anytype of data such as video, audio, tele-text, data, video/audio clips,and so on. Terminal 110 may be interested in receiving one or morespecific data channels from broadcast system 130. Terminal 110 mayascertain where each desired data channel is sent, e.g., based on theoverhead information and/or the data sent on the data channel. Terminal110 may go to sleep much of the time to conserve battery power and maywake up periodically to receive the desired data channel(s).

In the embodiment shown in FIG. 2, data is transmitted using OrthogonalFrequency Division Multiplexing (OFDM). OFDM partitions the systembandwidth into multiple (K) orthogonal subcarriers, which are alsocalled tones, bins, and so on. Each subcarrier may be modulated withdata. Each frame carries multiple (M) OFDM symbols. An OFDM symbol maybe generated by (1) performing a K-point IFFT on K modulation symbols toobtain K time-domain samples for a data portion of the OFDM symbol and(2) copying the last C samples of the data portion to form a cyclicprefix for the OFDM symbol. The data portion is also referred to as auseful portion, a transformed symbol, and so on. Windowing/filtering mayalso be performed on the cyclic prefix and the data portion. An OFDMsymbol may contain K+C samples without windowing or possibly more thanK+C samples with windowing.

In an embodiment, K=4096, C=512, and each OFDM symbol contains 4608time-domain samples prior to windowing. In an embodiment, L=128, S=36,and the TDM pilot contains 36 identical pilot sequences of length 128.Other values may also be used for K, C, L and S.

FIG. 2 shows a specific super-frame structure. The frequency controltechniques described herein may also be used for other frame andsuper-frame structures.

FIG. 3 shows a block diagram of an embodiment of terminal 110. In thisembodiment, terminal 110 includes an antenna 310 a and a receiver 320 ato receive signals from the cellular system, an antenna 310 b and areceiver 320 b to receive signals from the broadcast system, and anantenna 310 c and a receiver 320 c to receive signals from GPSsatellites. In general, terminal 110 may include any number of antennasand any number of receivers for any number of systems. Multiple systemsmay share an antenna if the antenna can provide suitable performance forthese systems. Multiple systems may also share a receiver if thesesystems are not received simultaneously. Multiple antennas and/ormultiple receivers may also be used for a given system, e.g., to receivesignals in different frequency bands (e.g., cellular and PCS bands).

For the broadcast system, antenna 310 b receives signals transmitted bybroadcast stations and provides a received RF signal to receiver 320 b.Within receiver 320 b, a low noise amplifier (LNA) 322 b amplifies thereceived RF signal and provides an amplified RF signal. A filter 322 bfilters the amplified RF signal to pass signal components in the band ofinterest and to remove out-of-band noise and undesired signals. Adownconverter 324 b frequency downconverts the filtered RF signal withan LO signal B_(LO) from an LO generator 344 and provides adownconverted signal. The frequency of the B_(LO) signal is selectedsuch that the signal component in an RF channel of interest isdownconverted to baseband or near-baseband. A lowpass filter 326 bfilters the downconverted signal to pass the signal components in the RFchannel of interest and to remove noise and undesired signals. Anamplifier 326 b amplifies the filtered baseband signal and provides anoutput baseband signal. An analog-to-digital converter (ADC) 328 bdigitizes the output baseband signal and provides input samples B_(in)to a data processor 330.

Antenna 310 a and receiver 320 a similarly receive and process signalstransmitted by base stations in the cellular system and provide inputsamples C_(in) to data processor 330. Antenna 310 c and receiver 320 creceive and process signals transmitted by GPS satellites and provideinput samples G_(in) to data processor 330. Although not shown in FIG. 3for simplicity, the input samples B_(in), C_(in) and G_(in) may becomplex-valued samples having inphase (I) and quadrature (Q) components.

FIG. 3 shows a specific design for receivers 320 a, 320 b and 320 c. Ingeneral, a receiver may implement a super-heterodyne architecture or adirect-to-baseband architecture. In the super-heterodyne architecture,the received RF signal is downconverted in multiple stages, e.g., fromRF to an intermediate frequency (IF) in one stage, and then from IF tobaseband in another stage. In the direct-to-baseband architecture, whichis shown in FIG. 3, the received RF signal is downconverted from RFdirectly to baseband in one stage. The super-heterodyne anddirect-to-baseband architectures may use different circuit blocks anddifferent LO frequencies.

In general, a receiver may perform signal conditioning with one or morestages of amplifier, filter, mixer, and so on. A receiver may includedifferent and/or additional circuit blocks not shown in FIG. 3.

Data processor 330 processes the input samples B_(in), C_(in) and G_(in)and provides output data for each system. The processing for each systemis dependent on the radio technology used by that system and may includedemodulation, decoding, and so on. Data processor 330 is shown as asingle processor in FIG. 3 but may comprise one or more applicationspecific integrated circuits (ASICs), digital signal processors (DSPs),processors, and so on.

An AFC unit 340 estimates frequency error of a reference oscillator 342and generates a frequency control signal F_(ctrl), as described below.Reference oscillator 342 generates a reference signal having a precisefrequency f_(ref). Reference oscillator 342 may be a voltage controlledcrystal oscillator (VCXO), a temperature compensated crystal oscillator(TCXO), a voltage controlled TCXO (VC-TCXO), a voltage controlledoscillator (VCO), or some other type of oscillator. LO generator 344receives the reference signal and generates the LO signals for receivers320 a, 320 b and 320 c. A clock generator 346 also receives thereference signal and generates sampling clocks for ADCs 328 a, 328 b and328 c. LO generator 344 and clock generator 346 may each be implementedwith VCOs, phase locked loops (PLLs), dividers, and so on, as is knownin the art.

A controller/processor 350 directs the operation of various units atterminal 110. Controller/processor 350 may determine which system(s) toreceive and which channel(s) to receive for each system. A memory 352stores program codes and data for terminal 110.

In the embodiment shown in FIG. 3, reference oscillator 342 provides thereference frequency f_(ref) that is used to generate the LO signals aswell as the sampling clocks for all systems. The use of a singlereference oscillator for all systems may reduce cost, power and area andmay further simplify operation at terminal 110.

Each transmitter in each system (e.g., each base station, each broadcaststation, and each satellite) typically transmits at a precise data rateon a precise RF frequency. The reference oscillator at the terminal isrelatively accurate but may have frequency error due to componenttolerances, temperature variations, and other factors. Furthermore,frequency error may exist between a given transmitter and the terminaldue to Doppler effect caused by movement of the terminal and/ortransmitter. Frequency error due to component tolerances and temperaturevariations typically varies at a slow rate and is common for allsystems. Frequency error due to Doppler effect may vary at a faster rateand may be different for different transmitters.

In an aspect, dual-loop AFC is used for frequency control at theterminal. The dual-loop AFC includes (1) an inner loop that correctsshort-term frequency variations, e.g., due to Doppler effect, and (2) anouter loop that corrects long-term frequency variations, e.g., due tocomponent tolerances and temperature variations. The dual-loop AFC maybe controlled such that good performance is achieved for all systemsbeing received, regardless of which systems are being received.

FIG. 4 shows a block diagram of an embodiment of AFC unit 340 in FIG. 3.In this embodiment, AFC unit 340 implements an inner loop and an outerloop for the cellular system and an inner loop and an outer loop for thebroadcast system. The reference oscillator is driven by either the outerloop for the cellular system or the outer loop for the broadcast system.

Within AFC unit 340, an inner loop unit 410 a receives the input samplesC_(in) for the cellular system, estimates the short-term frequency errorbetween a base station and the terminal, corrects this frequency errorin the input samples C_(in) and provides output samples C_(out) to acellular demodulator (Demod) 450 a, and further provides a short-termfrequency error estimate F_(Cin) for the cellular system. Similarly, aninner loop unit 410 b receives the input samples B_(in) for thebroadcast system, estimates the short-term frequency error between abroadcast station and the terminal, corrects this frequency error in theinput samples B_(in) and provides output samples B_(out) to a broadcastdemodulator 450 b, and further provides a short-term frequency errorestimate F_(Bin) for the broadcast system. Units 410 a and 410 bimplement the inner loops for the cellular and broadcast systems,respectively.

An outer loop unit 420 a receives the short-term frequency errorestimate F_(Cin) for the cellular system, estimates the long-termfrequency error for the terminal, and provides a long-term frequencyerror estimate F_(Cout) to a mode selector 430. Similarly, an outer loopunit 420 b receives the short-term frequency error estimate F_(Bin) forthe broadcast system, estimates the long-term frequency error for theterminal, and provides a long-term frequency error estimate F_(Bout) tomode selector 430. Selector 430 selects either the F_(Cout) or F_(Bout)frequency error estimate based on a Mode_sel control signal andgenerates the F_(ctrl) control signal for the reference oscillator. TheF_(ctrl) signal may be an analog signal or a digital signal. Selector430 may perform signal conditioning such as digital-to-analogconversion, level shifting, scaling, and so on. Selector 430 may alsogenerate a pulse width modulated (PWM) control signal.

FIG. 4 shows an embodiment in which two outer loops are maintained fortwo systems, and the reference oscillator is adjusted based on the outerloop for one system. In another embodiment, a single outer loop ismaintained for both systems, and the reference oscillator is adjustedbased on this single outer loop. This outer loop may receive the F_(Bin)and F_(Cin) frequency error estimates from the inner loops for the twosystems and may generate a single F_(out) based on these frequency errorestimates.

Table 1 lists two modes of operation for the terminal. In the cellularand broadcast mode, the terminal concurrently receives the cellular andbroadcast systems. In the broadcast-only mode, the terminal receives thebroadcast system but not the cellular system. The terminal may alsooperate in a cellular-only mode (not shown in Table 1) in which theterminal receives the cellular system but not the broadcast system. Theterminal may also receive GPS in any operating mode.

Table 1 also lists an embodiment of operating the inner and outer loopsfor each operating mode. In the cellular and broadcast mode, the innerloops for the cellular and broadcast systems are enabled and track theshort-term frequency errors for these systems, and only the outer loopfor the cellular system is enabled to control the reference oscillator.In the broadcast-only mode, the inner and outer loops for the cellularsystem are disabled, the inner and outer loops for the broadcast systemare enabled, and the reference oscillator is controlled by the outerloop for the broadcast system.

TABLE 1 Cellular and Broadcast Broadcast-Only Control of referenceoscillator from cellular from broadcast outer loop outer loop Cellularinner loop Enabled Disabled Cellular outer loop Enabled DisabledBroadcast inner loop Enabled Enabled Broadcast outer loop DisabledEnabled

FIG. 4 and Table 1 show an embodiment in which the reference oscillatoris controlled by either the cellular system or the broadcast system. Inother embodiments, the reference oscillator may be controlled bydifferent and/or additional systems. For example, if the terminalreceives GPS but not cellular or broadcast, then the referenceoscillator may be controlled by an AFC unit that operates based on GPSsignals. For clarity, the following description is for the embodimentshown in FIG. 4 and Table 1.

The inner and outer loops for the cellular and broadcast systems may beimplemented in various manners. An exemplary design for the inner andouter loops for the broadcast system is described below.

FIG. 5 shows a block diagram of an embodiment of inner loop unit 410 band outer loop unit 420 b for the broadcast system. Within inner loopunit 410 b, the input samples B_(in) are provided to a phase rotator 510and an initial frequency error estimator 520. Estimator 520 derives aninitial frequency error estimate Δ{circumflex over (f)}_(init) (e.g.,based on the TDM pilot) whenever directed and provides the initialfrequency error estimate to one input of a multiplexer (Mux) 546. Phaserotator 510 rotates each input sample B_(in) by a phase value θ_(k) froma phase accumulator 512 and provides a phase-rotated output sampleB_(out). The output samples have much of the short-term frequency errorremoved once frequency lock is achieved for the broadcast system. Afrequency error estimator 530 derives frequency error estimatesΔ{circumflex over (f)}_(m), e.g., based on received OFDM symbols. Thefrequency error estimates are indicative of residual frequency error inthe output samples. A frequency lock detector 532 determines whetherfrequency lock is achieved for the broadcast system.

A loop filter 540 filters the frequency error estimates Δ{circumflexover (f)}_(m) and provides F_(Bin), which is indicative of theshort-term frequency error for the broadcast system. Within loop filter540, a multiplier 542 multiplies the frequency error estimatesΔ{circumflex over (f)}_(m) with an inner loop gain α. A summer 544 sumsthe output of multiplier 542 with the output of a frequency register548. Multiplexer 546 receives the output of summer 544 at another inputand provides either the output of summer 544 or the initial frequencyerror estimate Δ{circumflex over (f)}_(init). Frequency register 548stores the output of multiplexer 546 and provides the short-termfrequency error estimate F_(Bin). Phase accumulator 512 accumulates theshort-term frequency error estimate in each sample period and providesthe phase correction for each input sample.

Within outer loop unit 420 b, a frequency accumulator 550 accumulatesthe frequency error estimate F_(Bin) from register 548 and provides anaccumulated frequency error. A counter 552 counts the number of timesthat F_(Bin) is accumulated in accumulator 550. A unit 554 divides theaccumulated frequency error by the number of accumulations and providesan average frequency error estimate Δ{circumflex over (f)}_(avg). A loopfilter 560 filters the average frequency error estimate and providesF_(Bout), which is indicative of the long-term frequency error in thereference frequency. Within loop filter 560, a multiplier 562 multipliesthe average frequency error estimate with an outer loop gain β. A unit564 may limit the output of multiplier 562 to within a particular rangeto constrain the amount of adjustment to the outer loop in any updateperiod. Unit 564 may also scale the output of multiplier 562. A summer566 sums the output of unit 564 with the output of a frequency register570. Frequency register 570 stores the output of summer 566 and providesthe long-term frequency error estimate F_(Bout).

The inner and outer loops for the broadcast system may operate asfollows. When the terminal first wakes up or first tunes to thebroadcast system, estimator 520 derives an initial frequency errorestimate Δ{circumflex over (f)}_(init) that captures much of theshort-term and long-term frequency error at the terminal. Frequencyregister 548 stores the initial frequency error estimate. Phaseaccumulator 512 computes the phase shift in each sample period due tothe frequency error from register 548. Phase rotator 510 rotates eachinput sample by the phase shift from phase accumulator 512. Thereafter,for each received OFDM symbol, estimator 530 derives a frequency errorestimate Δ{circumflex over (f)}_(m) based on the output samples for thatOFDM symbol. The frequency error estimate Δ{circumflex over (f)}_(m) isscaled by the inner loop gain α and accumulated by frequency register548 via summer 544 and multiplexer 546. Hence, frequency register 548 isinitialized with the initial frequency error estimate and is thereafterupdated by the frequency error estimate from each received OFDM symbol.

In an embodiment, outer loop 420 b is updated in each frame. Frequencyaccumulator 550, counter 552 and frequency register 570 are reset tozero at the start of each frame. Thereafter, frequency accumulator 550accumulates the output of frequency register 548 in each OFDM symbolperiod and up to M times in one frame for the super-frame structureshown in FIG. 2. Counter 552 increments by one each time the output ofregister 548 is accumulated by accumulator 550. At the end of eachframe, unit 554 computes the average frequency error estimateΔ{circumflex over (f)}_(avg), which is then scaled by the outer loopgain β, limited and/or scaled by unit 564, and accumulated by frequencyregister 570 via summer 566. Frequency register 570 is updated by ascaled version of the average frequency error estimate in each frame.

In the embodiment described above, phase rotation is performed on eachinput sample, the inner loop is updated in each OFDM symbol period, andthe outer loop is updated in each frame. The inner and outer loops mayalso be updated at other rates. In general, the inner loop may beupdated whenever a frequency error estimate is available, and the outerloop may be updated whenever an average frequency error estimate isavailable. For example, the outer loop may be updated after receiving aburst of data. The inner and outer loops may also be operated indifferent modes, e.g., an acquisition mode and a tracking mode, asdescribed below.

The input samples for the broadcast system may be expressed as:x(k)=s(k)·e ^(j2π·Δf·k·T) ^(s) ^(+φ) +n(k),  Eq (1)where s(k) is a sample transmitted in sample period k, x(k) is an inputsample for sample period k, n(k) is the noise for input sample x(k), Δfis a frequency error, φ is an arbitrary phase, and T_(s) is one sampleperiod.

The TDM pilot contains S identical pilot sequences, as shown in FIG. 2.Hence, the transmitted samples are periodic during the TDM pilot, ands(k)=s(k+L). In this case, a correlation on the input samples may beexpressed as:x*(k)·x(k+L)=|s(k)|² ·e ^(j2π·Δf·L·T) ^(s) +ñ(k),  Eq (2)where ñ(k) is the post-processed noise. Equation (2) indicates that thefrequency error Δf may be isolated by correlating input sample x(k) withdelayed input sample x(k+L).

A delayed correlation may be performed for each pilot sequence asfollows:

$\begin{matrix}{{C_{l} = {\sum\limits_{i = 1}^{L}{{x_{l}^{*}(i)} \cdot {x_{l}\left( {i + L} \right)}}}},} & {{Eq}\mspace{14mu}(3)}\end{matrix}$where x_(l)(i)=x(i+l·k_(s)) is the i-th input sample for the l-th pilotsequence,

-   -   k_(s) is the sample index for the start of the first pilot        sequence, and    -   C_(l) is the correlation result for the l-th pilot sequence.

The correlation results for multiple pilot sequences may be accumulated,as follows:

$\begin{matrix}{{C_{init} = {\sum\limits_{l = 1}^{S^{\prime}}C_{l}}},} & {{Eq}\mspace{14mu}(4)}\end{matrix}$where S′ is the number of delayed correlations performed, which is S′<S,and

-   -   C_(init) is the accumulated correlation result for all pilot        sequences.        Equation (4) performs coherent accumulation on the S′        correlation results and provides C_(init), which is a complex        value.

An initial frequency error estimate may then be derived based on theaccumulated correlation result, as follows:

$\begin{matrix}{{{\Delta\;{\hat{f}}_{init}} = {\frac{1}{G_{L}}\;\arctan\;\left( C_{init} \right)}},} & {{Eq}\mspace{14mu}(5)}\end{matrix}$where G_(L) is a detector gain, which is G_(L)=2π·L·T_(s).

The start of the first pilot sequence may be ascertained by performing asliding correlation on the input samples and detecting for a peak in thesliding correlation. The input samples may be buffered, and the delayedcorrelation in equation (3) may be performed for all pilot sequencesafter the TDM pilot has been detected. Alternatively, the TDM pilot maybe detected using some of the pilot sequences, and the initial frequencyerror estimate may be derived using the remaining pilot sequences.

FIG. 6 shows a block diagram of an embodiment of initial frequency errorestimator 520 in FIG. 5. In this embodiment, a delayed correlator 610receives the input samples B_(in) for the broadcast system and performsthe delayed correlation shown in equation (4). Within delayed correlator610, the input samples are provided to an L-sample delay line 612 and amultiplier 616. Delay line 612 delays each input sample by L sampleperiods, which is the length of the pilot sequence. A unit 614 providesthe complex conjugate of each delayed sample from delay line 612.Multiplier 616 multiplies each input sample with the correspondingoutput from unit 614 and provides the product x*_(l)(i)·x_(l)(i+L) ineach sample period. A peak detector 620 detects for the TDM pilot andprovides sample index k_(s) for the start of the first pilot sequence.An accumulator 618 accumulates the output of multiplier 616 over Lsample periods for one pilot sequence and provides the correlationresult C_(l) for each pilot sequence.

An accumulator 630, which is formed with a summer 632 and a register634, accumulates the correlation results from delayed correlator 610 forS′ pilot sequences and provides the accumulated result C_(init). Anarctan unit 640 computes the arctangent of C_(init). A scaling unit 642scales the output of arctan unit 640 and provides the initial frequencyerror estimate Δ{circumflex over (f)}_(init).

Each OFDM symbol contains a cyclic prefix that is identical to the lastC samples of the data portion, as shown in FIG. 2. Hence, the samplesduring the cyclic prefix are periodic, so that s(k)=s(k+K). A frequencyerror estimate may be computed for each OFDM symbol based on the cyclicprefix, as follows:

$\begin{matrix}{{{\Delta\;{\hat{f}}_{m}} = {{Im}\;\left\lbrack {\sum\limits_{i = 1}^{C^{\prime}}{{y_{m}^{*}(i)} \cdot {y_{m}\left( {i + K} \right)}}} \right\rbrack}},} & {{Eq}\mspace{14mu}(6)}\end{matrix}$where y_(m)(i) is the i-th output sample for the m-th OFDM symbol,

-   -   Δ{circumflex over (f)}_(m) is a frequency error estimate for the        m-th OFDM symbol, and    -   C′ is the number of samples over which correlation is performed,        where C′≦C.        The imaginary part Im [ ] in equation (6) is an approximation of        the arctangent in equation (5). This approximation is reasonably        accurate when the quantity within the square bracket in        equation (6) is small, which is typically the case once        frequency lock is achieved.

The start of each OFDM symbol may be determined by a time tracking loopknown in the art and not described herein. The accumulation in equation(6) may be performed over all or a subset of the C samples for thecyclic prefix.

FIG. 7 shows a block diagram of an embodiment of frequency errorestimator 530 in FIG. 5. In this embodiment, a delayed correlator 710receives the output samples B_(out) for the broadcast system andperforms the delayed correlation shown within the square bracket inequation (6). Delayed correlator 710 includes a delay line 712, acomplex-conjugate unit 714, a multiplier 716, and an accumulator 718that operate in similar manner as units 612, 614, 616 and 618,respectively, within delayed correlator 610 in FIG. 6. However, delayline 712 delays each output sample by K sample periods, which is thelength of the data portion. Accumulator 718 accumulates the output ofmultiplier 716 over C′ sample periods for the cyclic prefix and providesa correlation result C_(m) for each OFDM symbol. A unit 720 provides theimaginary part of the correlation result C_(m) as the frequency errorestimate Δ{circumflex over (f)}_(m).

FIGS. 6 and 7 show exemplary embodiments of frequency error estimators520 and 530, respectively. The embodiment in FIG. 6 relies on theperiodic nature of the TDM pilot to derive the initial frequency errorestimate. The embodiment in FIG. 7 relies on the periodic nature of thecyclic prefix in each OFDM symbol to derive a frequency error estimate.In general, frequency error estimation may be performed in variousmanners depending on the structure of the transmitted signal, the radiotechnology used for the transmitted signal, and/or other factors.

Referring back to FIG. 5, frequency register 548 provides the currentfrequency error estimate F_(Bin)=Δ{circumflex over (f)} for thebroadcast system. Phase accumulator 512 accumulates this frequency errorestimate in each sample period and provides a phase value, which may begiven as θ_(k)=−2π·k·Δ{circumflex over (f)}. Phase rotator 510 mayrotate each input sample as follows:y ₁(k)+jy _(Q)(k)=[x ₁(k)+jx _(Q)(k)]·[cos θ_(k) +j sin θ_(k)]  Eq (7)where x(k)=x₁(k)+jx_(Q)(k) is a complex-valued input sample for sampleperiod k, and

-   -   y(k)=y₁(k)+jy_(Q)(k) is a complex-valued output sample for        sample period k.        The complex multiplication in equation (7) may be performed with        four real multiplications and two real additions.

Frequency lock detector 532 may detect for frequency lock in variousmanners. In an embodiment, detector 532 initially resets a counter tozero. Thereafter, detector 532 compares each frequency error estimateΔ{circumflex over (f)}_(m) from estimator 530 against a thresholdΔf_(th), increments the counter if the frequency error estimate is lessthan the threshold, and decrements the counter otherwise. Detector 532declares frequency lock if the counter reaches a maximum value anddeclares loss of lock if the counter reaches zero. The number of bitsfor the counter and the threshold Δf_(th) may be selected to achievegood lock detection performance. Frequency lock may also be detected inother manners.

In an embodiment, the AFC for the broadcast system may be operated in anacquisition mode or a tracking mode. Both loop modes may be applicablewhen receiving both cellular and broadcast or when receiving onlybroadcast. For clarity, the following description is for the case whenreceiving only broadcast.

In an embodiment, the inner and outer loops are both operational in theacquisition and tracking modes, and different parameter values may beused for the inner and/or outer loop in the two modes. For the innerloop, the same inner loop gain a may be used for both modes.Alternatively, a larger inner loop gain may be used for the acquisitionmode, and a smaller inner loop gain may be used for the tracking mode.For the outer loop, a larger outer loop gain β and/or a larger limit maybe used in the acquisition mode, and frequency register 570 may beupdated by a larger amount in each update interval. In the trackingmode, a smaller outer loop gain β and/or a smaller limit may be used,and frequency register 570 may be updated by a smaller amount in eachupdate interval.

In another embodiment, the outer loop adjusts the reference oscillatorto correct short-term and long-term frequency error in the acquisitionmode, and the inner and outer loops are both operational in the trackingmode. In the acquisition mode, the inner loop derives frequency errorestimates based on the input samples (and not the output samples) andprovides these frequency error estimates to the outer loop. The outerloop drives the reference oscillator to the correct frequency. The outerloop may be operated with a larger outer loop gain β and/or a largerlimit in the acquisition mode. In this embodiment, the inner loop isessentially non-operational in the acquisition mode, and the outer loopattempts to quickly move the reference oscillator to the correctfrequency. In the tracking mode, the outer loop slowly updates thereference oscillator, and the inner loop corrects for short-termfrequency error.

The acquisition and tracking modes may also be implemented in othermanners. The terminal may support different and/or additional modes. Forexample, the terminal may also support a hold mode in which the innerand/or outer loops are maintained fixed, e.g., if the received signalquality is poor or if some other conditions are detected.

The terminal may start in the acquisition mode when powered on, afterwaking up from an extended sleep, when frequency lock is lost, and/orfor other conditions. The terminal may transition from the acquisitionmode to the tracking mode upon detecting frequency lock, if theadjustment applied to frequency register 570 is below a particular valuefor some number of updates, and/or if some other conditions aresatisfied.

The terminal may periodically receive data from the broadcast system.For example, each frame may carry many OFDM symbols (e.g., approximately300 OFDM symbols), and the terminal may receive only few OFDM symbols(if any) in each frame. In this case, the terminal may sleep for most ofthe frame, wake up several OFDM symbols prior to the first OFDM symbolof interest, and process each OFDM symbol of interest. The terminal mayupdate the inner loop in each OFDM symbol period while awake and mayupdate the outer loop prior to going to sleep.

FIG. 8 shows a block diagram of a model 800 of the dual-loop AFC for thebroadcast system. Model 800 includes a section 810 for the inner loopand a section 820 for the outer loop. Model 800 represents the operationof the inner and outer loops during the tracking mode.

Within outer loop section 820, a summer 822 subtracts the referencefrequency f_(ref) from a received frequency f_(rx) and provides an inputfrequency f_(in). The received frequency is the frequency of a signalreceived from the broadcast system, the reference frequency is thefrequency of the reference oscillator, and the input frequency is thefrequency error of the input samples B_(in). Summer 822 models thefrequency downconversion by downconverter 324 b in FIG. 3.

Within inner loop section 810, a summer 812 subtracts a rotatorfrequency f_(rot) from the input frequency f_(in) and provides afrequency error f_(err). Summer 812 models the phase rotation by unit510 in FIG. 5. The rotator frequency f_(rot) is the frequency providedby register 548, and the frequency error f_(err) is the residualfrequency error estimated by frequency error estimator 530 in FIG. 5.The frequency error f_(err) is scaled with the inner loop gain α by amultiplier 816 and accumulated by an accumulator 814. Multiplier 816corresponds to multiplier 532 in FIG. 5, and accumulator 814 correspondsto summer 544 and frequency register 548. Accumulator 814 has a transferfunction of 1/(z−1) in the z-domain.

In outer loop section 820, the rotator frequency f_(rot) is scaled withthe outer loop gain β by a multiplier 826 and accumulated by anaccumulator 824 to generate the reference frequency. Multiplier 826corresponds to multiplier 562 in FIG. 5, and accumulator 824 correspondsto summer 566 and frequency register 570.

A transfer function H_(in)(z) for the inner loop may be expressed as:

$\begin{matrix}{{H_{in}(z)} = {\frac{f_{rot}}{f_{in}} = {\frac{\alpha}{z - 1 + \alpha}.}}} & {{Eq}\mspace{14mu}(8)}\end{matrix}$

A transfer function H_(out)(z) for the outer loop may be expressed as:

$\begin{matrix}{{H_{out}(z)} = {\frac{f_{ref}}{f_{rx}} = {\frac{\alpha \cdot \beta}{\left( {z - 1} \right)^{2} + {\left( {z - 1} \right) \cdot \alpha} + {\alpha \cdot \beta}}.}}} & {{Eq}\mspace{14mu}(9)}\end{matrix}$

Since the sampling rate is typically much higher than the inner andouter loop bandwidths, the z-domain transfer functions in equations (8)and (9) may be converted to s-domain transfer functions using theapproximation z−1=jω=s, where ω is normalized frequency. The s-domaintransfer functions may be expressed as:

$\begin{matrix}\begin{matrix}{{H_{in}(s)} = \frac{\alpha}{s + \alpha}} & {and} & {{H_{out}(s)} = {\frac{\alpha \cdot \beta}{s^{2} + {\alpha \cdot s} + {\alpha \cdot \beta}}.}}\end{matrix} & {{Eq}\mspace{14mu}(10)}\end{matrix}$

The bandwidth of the inner loop may be expressed as:

$\begin{matrix}{{{BW}_{in} = {\frac{\alpha}{4} = \frac{\xi \cdot \omega_{n}}{2}}},} & {{Eq}\mspace{14mu}(11)}\end{matrix}$where ω_(n)=√{square root over (α·β)} is a natural frequency of theloop, and

-   -   ξ=0.5√{square root over (α/β)} is a damping factor for the loop.

The bandwidth of the outer loop may be expressed as:BW _(out)=ω_(n)·[(1−2ξ²)+√{square root over (4ξ⁴−4ξ²+2)}]^(1/2).  Eq(12)

The outer loop bandwidth is typically much more narrow than the innerloop bandwidth in the tracking mode. The inner and outer loop bandwidthsmay be determined as follows. A desired inner loop bandwidth and adesired ratio of BW_(in) to BW_(out) are initially selected. The dampingfactor ξ is then determined based on the ratio of BW_(in) to BW_(out)using equations (11) and (12). The natural frequency ω_(n) is nextdetermined based on the damping factor ξ and the inner loop bandwidthBW_(in) using equation (11). The inner loop gain α is determined basedon the inner loop bandwidth using equation (11). The outer loop gain βis determined based on the inner loop gain α and the natural frequencyω_(n). In one exemplary design, BW_(in)=128 Hertz (Hz), BW_(out)=12.8Hz, ξ=3.2, ω_(n)=0.062, α=0.4 and β=0.01. Other designs may also be usedfor the inner and outer loops. In general, the inner and outer loops maybe designed to achieve the desired frequency acquisition and trackingperformance for the specified operating scenarios.

For clarity, the inner and outer loops have been described for aspecific broadcast system. Other designs may also be used for the innerand outer loops for the broadcast system. The inner and outer loops forthe cellular system and/or other systems may be designed in accordancewith the structure of the signals transmitted by these systems and theradio technologies used by these systems. For example, frequency errorestimates may be derived based on a pilot transmitted by a system. Thepilot may be transmitted continuously or periodically, and the innerloop may be updated whenever the pilot is received.

FIG. 9 shows an embodiment of a process 900 for performing frequencycontrol for multiple communication systems. Frequency control for afirst communication system is performed with a first inner loop (block912). Frequency control for a second communication system is performedwith a second inner loop (block 914). A reference frequency used toreceive the first and second communication systems is adjusted with anouter loop (block 916). The first system may be a broadcast system, andthe second system may be a cellular system. The first and second systemsmay utilize two different radio technologies. For example, the firstsystem may be a broadcast system that utilizes OFDM, and the secondsystem may be a CDMA system. The reference frequency may also be used toreceive a satellite positioning system, e.g., GPS. The referencefrequency may be used for frequency downconversion, sampling and/orother purposes.

The first inner loop estimates and corrects frequency error in a firstinput signal for the first system. The second inner loop estimates andcorrects frequency error in a second input signal for the second system.The first and second inner loops may be enabled when receiving the firstand second systems, respectively.

In an embodiment, first and second outer loops are implemented for thefirst and second systems, respectively, and are updated with the firstand second inner loops, respectively. The first inner loop and the firstouter loop may be operational when receiving only the first system. Thefirst and second inner loops and the second outer loop may beoperational when receiving the first and second systems. In anotherembodiment, a single outer loop is implemented for both systems and isupdated with the first inner loop or the second inner loop, or bothinner loops. In general, the outer loop that is operational estimatesthe frequency error between the reference frequency and the frequency ofthe first and/or second system and updates the reference frequency.

FIG. 10 shows an embodiment of an apparatus 1000 for performingfrequency control for multiple communication systems. Apparatus 1000includes means for performing frequency control for a firstcommunication system with a first inner loop (block 1012), means forperforming frequency control for a second communication system with asecond inner loop (block 1014), and means adjusting a referencefrequency, used to receive the first and second communication systems,with an outer loop for (block 916).

FIG. 11 shows an embodiment of a process 1100 for performing frequencycontrol for one communication system. Frequency error for a firstcommunication system (e.g., a broadcast system) is estimated andcorrected with an inner loop (block 1112). Frequency error in areference frequency used to receive the first system and a secondcommunication system (e.g., a cellular system) is estimated andcorrected with an outer loop (block 1114). The inner loop is enabledwhen receiving the first system (block 1116). The outer loop is enabledwhen receiving the first system and if this loop is designated to adjustthe reference frequency (block 1118).

The inner loop may comprise a phase rotator, first and second frequencyerror estimators, a loop filter, a frequency lock detector, or acombination thereof. The phase rotator corrects frequency error in inputsamples for the first system and provides output samples. The firstfrequency error estimator derives frequency error estimates indicativeof the residual frequency error in the output samples. The firstfrequency error estimator may derive a frequency error estimate for eachreceived OFDM symbol by correlating the cyclic prefix with the dataportion. The second frequency error estimator derives an initialfrequency error estimate indicative of the frequency error in the inputsamples. The second frequency error estimator may derive the initialfrequency error estimate by correlating periodic sequences in the signalreceived from the first system. The inner loop filter may be initializedwith the initial frequency error estimate and may thereafter filter thefrequency error estimates from the first frequency error estimator togenerate an output for the inner loop. The frequency lock detectordetermines whether frequency lock is achieved for the first system.

The outer loop may comprise first and second modules, a loop filter, ora combination thereof. The first module computes an average frequencyerror from the inner loop. The second module limits the inputs for theouter loop filter. The outer loop filter filters the average frequencyerror and provides an output for the outer loop. The outer loop may beoperable in an acquisition mode or a tracking mode. In the acquisitionmode, the second module may limit the inputs for the outer loop filterto within a first range and/or the outer loop filter may use a firstgain value. In the tracking mode, the second module may limit the inputsfor the outer loop filter to within a second range that is smaller thanthe first range and/or the outer loop filter may use a second gain valuethat is smaller than the first gain value.

FIG. 12 shows an embodiment of an apparatus 1200 for performingfrequency control for one communication system. Apparatus 1200 includesmeans for estimating and correcting frequency error for a firstcommunication system with an inner loop (block 1212), means forestimating and correcting frequency error in a reference frequency withan outer loop (block 1214), means for enabling the inner loop whenreceiving the first system (block 1216), and means for enabling theouter loop when receiving the first system and if this loop isdesignated to adjust the reference frequency (block 1218).

FIG. 13 shows an embodiment of a process 1300 for performing frequencycontrol for a bursty transmission. A terminal receives data in bursts,e.g., from a broadcast system, a cellular system, or some other system(block 1312). The terminal updates an inner loop for AFC in each innerloop update instant during each burst of data (block 1314). The innerloop estimates and corrects the frequency error in the bursts of data.The terminal updates an outer loop for AFC in each outer loop updateinstant (block 1316). The outer loop estimates and corrects thefrequency error in a reference frequency used to receive the bursts ofdata. The terminal may receive at least one OFDM symbol in each burst ofdata and may update the inner loop with each received OFDM symbol. Theterminal may update the outer loop at the end of each burst of data, ormore or less often. The terminal may wake up prior to each burst of dataand may sleep between bursts of data.

FIG. 14 shows an embodiment of an apparatus 1400 for performingfrequency control for a bursty transmission. Apparatus 1400 includesmeans for receiving data in bursts (block 1412), means for updating aninner loop for AFC in each inner loop update instant during each burstof data (block 1414), and means for updating an outer loop for AFC ineach outer loop update instant (block 1416).

The frequency control techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, firmware, software, or a combination thereof. For a hardwareimplementation, the processing units used for frequency control may beimplemented within one or more ASICs, DSPs, digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof.

For a firmware and/or software implementation, the techniques may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The firmware and/or softwarecodes may be stored in a memory (e.g., memory 352 in FIG. 3) andexecuted by a processor (e.g., processor 350). The memory may beimplemented within the processor or external to the processor.

In an embodiment, the inner loop is implemented in hardware, and theouter loop is implemented in software and/or firmware. In otherembodiments, the inner and outer loops may be implemented with othercombinations of hardware, software, and/or firmware.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An apparatus comprising: at least one processor configured to performfrequency control for short term frequency variations for a firstcommunication system with a first inner loop, to perform frequencycontrol for short term frequency variations for a second communicationsystem with a second inner loop, and to adjust a reference frequency forlong term frequency variations with an outer loop, the referencefrequency being used to receive at least one signal from the first andsecond communication systems, wherein the at least one processor isconfigured to implement first and second outer loops for the first andsecond communication systems, respectively, to update the first andsecond outer loops with relation to a parameter of the first and secondinner loops, respectively, and to select the first or second outer loopas the outer loop that adjusts the reference frequency; and a memorycoupled to the at least one processor.
 2. The apparatus of claim 1,wherein the at least one processor is configured to estimate and correctfrequency error in a first input signal for the first communicationsystem with the first inner loop, and to estimate and correct frequencyerror in a second input signal for the second communication system withthe second inner loop.
 3. The apparatus of claim 1, wherein the at leastone processor is configured to estimate and correct frequency errorbetween the reference frequency and a frequency of the first or secondcommunication system with the outer loop.
 4. The apparatus of claim 1,wherein the at least one processor is configured to enable the firstinner loop and the first outer loop when receiving only the firstcommunication system, and to enable the first and second inner loops andthe second outer loop when receiving the first and second communicationsystems.
 5. The apparatus of claim 1, wherein the at least one processoris configured to update the outer loop with the first inner loop or thesecond inner or both the first and second inner loops.
 6. The apparatusof claim 1, wherein the first communication system is a broadcast systemand the second communication system is a cellular system.
 7. Theapparatus of claim 1, wherein the first and second communication systemsutilize two different radio technologies.
 8. The apparatus of claim 1,wherein the first communication system is a broadcast system utilizingOrthogonal Frequency Division Multiplexing (OFDM), and wherein thesecond communication system is a Code Division Multiple Access (CDMA)system.
 9. The apparatus of claim 1, wherein the reference frequency isused for frequency downconversion of signals received from the first andsecond communication systems.
 10. The apparatus of claim 1, wherein thereference frequency is used for sampling signals for the first andsecond communication systems.
 11. The apparatus of claim 1, wherein thereference frequency is further used to receive a satellite positioningsystem.
 12. The apparatus of claim 1, wherein the short term frequencyvariations of at least of the first and second communication systemsarise from Doppler shifts.
 13. The apparatus of claim 12, wherein thelong term frequency variations comprise frequency variations due to atleast one of the following: component tolerances and temperaturevariations.
 14. The apparatus of claim 1, wherein the long termfrequency variations comprise frequency variations due to at least oneof the following: component tolerance and temperature variations. 15.The apparatus of claim 1, wherein at least one of the first inner loopand the second inner loop is configured as a scaling operation.
 16. Aprocessor configured to: perform frequency control for short termfrequency variations for a first communication system with a first innerloop, to perform frequency control for short term frequency variationsfor a second communication system with a second inner loop, and toadjust a reference frequency for long term frequency variations with anouter loop, the reference frequency being used to receive at least onesignal from the first and second communication systems; and update theouter loop with relation to a parameter of the first inner loop when thefirst communication system is selected to adjust the referencefrequency, and to update the outer loop with relation to a parameter ofthe second inner loop when the second communication system is selectedto adjust the reference frequency.
 17. The processor of claim 16, andfurther configured to enable the first inner loop when receiving thefirst communication system, and to enable the second inner loop whenreceiving the second communication system.
 18. The processor of claim16, wherein the short term frequency variations of at least of the firstand second communication systems arise from Doppler shifts.
 19. Theprocessor of claim 18, wherein the long term frequency variationscomprise frequency variations due to at least one of the following:component tolerances and temperature variations.
 20. The processor ofclaim 13, wherein the long term frequency variations comprise frequencyvariations due to at least one of the following: component tolerancesand temperature variations.
 21. The processor of claim 16, wherein atleast one of the first inner loop and the second inner loop isconfigured as a scaling operation.
 22. A method comprising: performingfrequency control for short term frequency variations for a firstcommunication system with a first inner loop; performing frequencycontrol for short term frequency variations for a second communicationsystem with a second inner loop; adjusting a reference for long termfrequency variations with an outer loop, the reference frequency beingused to receive at least one signal from the first and secondcommunication systems; updating the outer loop with relation to aparameter of the first inner loop when the first communication system isselected to adjust the reference frequency; and updating the outer loopwith relation to a parameter of the second inner loop when the secondcommunication system is selected to adjust the reference frequency. 23.The method of claim 22, further comprising: enabling the first innerloop when receiving a signal of the first communication system; andenabling the second inner loop when receiving a signal of the secondcommunication system.
 24. The method of claim 22, wherein the short termfrequency variations of at least one of the first and secondcommunication systems arise from Doppler shifts.
 25. The method of claim24, wherein the long term frequency variations comprise frequencyvariation due to at least one of the following: component tolerances andtemperature variations.
 26. The method of claim 22, wherein the longterm frequency variations comprise frequency variations due to at leastone of the following: component tolerances and temperature variations.27. The method of claim 22, wherein at least one of the first inner loopand the second inner loop is configured as a scaling operation.
 28. Anapparatus comprising: means for performing frequency control for shortterm frequency variations for a first communication system with a firstinner loop; means for performing frequency control for short termfrequency variations for a second communication system with a secondinner loop; means for adjusting a reference frequency for long termfrequency variations with an outer loop, the reference frequency beingused to receive at least one signal from the first and secondcommunication systems; means for updating the outer loop with aparameter of the first inner loop when the first communication system isselected to adjust the reference frequency; and means for updating theouter loop with a parameter of the second inner loop when the secondcommunication system is selected to adjust the reference frequency. 29.The apparatus of claim 28, further comprising: means for enabling thefirst inner loop when receiving the first communication system; andmeans for enabling the second inner loop when receiving the secondcommunication system.
 30. The apparatus of claim 28, wherein the shortterm frequency variations of at least one of the first and secondcommunication systems arise from Doppler shifts.
 31. The apparatus ofclaim 30, wherein the long term frequency variations comprise frequencyvariations due to at least one of the following: component tolerancesand temperature variations.
 32. The apparatus of claim 28, wherein thelong term frequency variations comprise frequency variations due to atleast one of the following: component tolerances and temperaturevariations.
 33. The apparatus of claim 28, wherein at least one of thefirst inner loop and the second inner loop is configured as a scalingoperation.
 34. A non-transitory computer-readable medium encoded with acomputer program, configured to: perform frequency control for shortterm frequency variations for a first communication system with a firstinner loop; perform frequency control for short term frequencyvariations for a second communication system with a second inner loop;adjust a reference frequency for long term frequency variations with anouter loop, the reference frequency being used to received at least onesignal from the first and second communication systems; update the outerloop with a parameter of the first inner loop when the firstcommunication system is selected to adjust the reference frequency; andupdate the outer loop with a parameter of the second inner loop when thesecond communication system is selected to adjust the referencefrequency.
 35. The computer-readable medium of claim 34, furtherconfigured to: enable the first inner loop when receiving the firstcommunication system; and enable the second inner loop when receivingthe second communication system.
 36. The computer-readable medium ofclaim 34, wherein the short term frequency variations of at least one ofthe first and second communication systems arise from Doppler shifts.37. The computer-readable medium of claim 36, wherein the long termfrequency variations comprise frequency variations due to at least oneof the following: component tolerances and temperature variations. 38.The computer-readable medium of claim 34, wherein the long termfrequency variations comprise frequency variations due to at least oneof the following: component tolerances and temperature variations. 39.The computer-readable medium of claim 34, wherein at least one of thefirst inner loop and the second inner loop is configured as a scalingoperation.
 40. A non-transitory computer-readable medium encoded with acomputer program, configured to: estimate and correct frequency errorfor short term frequency variations for a first communication systemwith an inner loop; estimate and correct frequency error in a referencefrequency for long term frequency variations with an outer loop, thereference frequency being used to receive at least one signal from thefirst communication system and a second communication system; and updatethe outer loop with a parameter of the inner loop when at least one ofthe first communication system and the second communication system isselected to adjusted the reference frequency.